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From Genes to Proteins - Translation
Ch. 17Sections 17.4, 17.5, 17.6, & 17.7
To assist you in your note taking…
Key vocabulary terms are in green, bold, underlined font
Overview of Concepts
1. The genetic code is a triplet code
2. Translation is directed by RNA molecules
3. RNA plays many different roles in protein synthesis
4. Point mutations may affect protein formation
The triplet codeThere are 20 amino
acids (the monomers of proteins) but only 4 nucleotides (the monomers of nucleic acids)
How can just 4 bases code for 20 different amino acids?
The triplet codeThe genetic code is
based on triplets of bases: a series of nonoverlapping, three nucleotide “words”
We call these base triplets in the mRNA codons
How did scientists figure out it was 3 bases for each codon?
The triplet code4 nucleotides (A,C,T,G) x 1 in a sequence =
4 different combinations
4 nucleotides x 2 in a sequence = 16 different combinations
4 different nucleotides x 3 in a sequence = 64 different combinations (for 20 AA’s) Many AA’s can be coded for up to 6 ways
The triplet codeThe code is redundant
but unambiguousEach codon codes for
only 1 amino acid - unambiguous
Some amino acids are coded for by more than one codon - redundant
Only UGG codes for tryptophan
AGU & AGC both code for serine
How did scientists figure out what amino acid each codon
codes for? 1960s - Nierenberg & Mathaei
Used artificial RNA triplets in tubes with the components for building proteins
Made chains of uracil first - UUUUUUUUUGot all phenylalanines in a chain,
so UUU must code for phenylalanine.
Within a few years, they had decoded all 64 codons
What is translation?
Translation is the process by which a cell interprets the codons along an mRNA molecule and builds a polypeptide
Who translates the code?
Transfer RNA (tRNA) is the interpreter of the genetic codetRNA is the molecule
responsible for converting the genetic code of nucleotides toto the protein code of amino acids
How does tRNA work?The cell already has all 20
amino acids in its cytoplasm (either makes them itself or they are taken in through the organism’s diet)
Each tRNA is a strand about 80 bases long
Some bases are complementary to each other so it can hydrogen bond to itselfTakes on a clover-leaf shape
tRNAOn one end of the
tRNA is an amino acid
On the other end is an anticodonThe anticodon is
complementary to the codon in the mRNA
So codon by codon, the tRNAs deposit amino acids in the prescribed order, and the ribosome joins them into a polypeptide chain
Some practice
DNA template strand:
ACCGGTCAGTAC1. Make the mRNA from this
template2. What will be the tRNA
anticodons?
RibosomesRibosomes are the sites
of protein synthesis
They are made up of ribosomal RNA (rRNA) & protein
Composed of 2 subunits: large & smallSubunits are made in the
nucleolusThey join together at the
mRNA to make a functional ribosome
RibosomesRibosomes bring
together the mRNA and the tRNAs bearing the correct amino acids and bond those amino acids in the correct order
There are 3 sites on the ribosome that function in this capacity: the E site, the P site, and the A site
A site - holds the tRNA with the next amino acid to be added to the chain
P site - holds the tRNA carrying the growing polypeptide chain
E site - releases tRNAs from the ribosome here
PA
Translation has 3 stages
InitiationElongationTermination
InitiationBrings together mRNA, the first
tRNA with the first amino acid, and the large & small subunits of the ribosome
The first amino acid is methionine (codon AUG, the start codon)
This establishes the reading frame
The whole thing is called a “translation initiation complex” and GTP energy is required to build it
Elongation More amino
acids are added to the growing chain
There are 3 steps catalyzed by protein elongation factors
STEP 1 - Codon Recognition
the anticodon on the tRNA H-bonds with the codon in the A site
1. 2 GTPs for energy are used up here
2. An elongation factor protein catalyzes this step
STEP 2 - Peptide Bond FormationThe large subunit catalyzes the formation of a
peptide bond between the amino acid in the A site and the amino acid in the P site
STEP 3 - Translocation
The ribosome moves the tRNA in the A site to the P site
The empty tRNA in the P site is moved to the E site and released
GTP energy is required here
TerminationHappens when one of the 3 stop
codons reaches the A site on the ribosome
A release factor protein binds to the stop codon & hydrolysis occurs to free the polypeptide chain
PolyribosomesSeveral ribosomes
can be working at the same mRNA strand at the same time
Strings of these ribosomes are called polyribosomes
This helps the cell make more proteins more quickly
ProteinsAs the polypeptide
chain is being formed, it will begin to coil & fold in to its 3-D shape
The gene determines the order of the amino acids - the primary structure
The primary structure determines the secondary and tertiary structure
ProteinsProteins may be further
modified by the addition of sugars, lipids, or phosphate groups
Enzymes may cleave the polypeptide chain into smaller chains
2 or more polypeptide chains may join to make the quaternary structure of a functional protein
ProteinsAll translation begins in the cytosol on
free ribosomesIf the protein is destined to become
part of an organelle or is to be shipped outside the cell, the ribosome will move to the ER and become an attached ribosome
ProteinsThere will be a signal peptide (a
sequence of amino acids) that is recognized by a protein-RNA complex called a signal recognition particle (SRP)
This particle brings the ribosome to the ER and translation continues there
Types of RNAmRNA - messenger RNA
(the code)tRNA - transfer RNA
(brings amino acids)rRNA - ribosomal RNA
(the ribosome)Pre-mRNA - the primary
transcript before editingsnRNA - part of
sliceosomesSRP RNA - part of the
signal recognition particle& others
What makes RNA so versatile?
1. It can H-bond to itself & to other nucleic acids
2. It has functional groups that allow it to act as an enzyme
Point MutationsA point mutation is a
change in a single base pair in a gene
They can have catastrophic consequence, or none at all
There are 3 main types:SubstitutionInsertionDeletion
Substitution mutations
A base pair is replaced with a different base pair
Because there is redundancy in the genetic code, this may cause no problem at all
It could also lead to a malformed protein and be the difference between life and death
Substitution
Think of it like a sentence:
Normal sentence would read THE DOG BIT THE CAT
A point mutation might make the sentence read:THE DOG BIT THE CAR
This changes the meaning of the sentence, but not dramatically.
Changing a single base can cause a dramatic change:The base change codes for a different
amino acid, making a different proteinExample: sickle cell anemia
Changing a single base may not cause any change at all:The changed base may
still code for the same amino acid
Proline is coded for by CCC, CCA, CCG, and CCU,
So a change in the last base won’t make any difference to the amino acid that is added to the protein chain.
Insertions & Deletions•These mutations add an extra letter or two or delete letters
•These mutations disrupt the reading frame and are usually more severe
•Because of this they are called frameshift mutations
Frameshift Mutations
Think of it as a sentence again:THE DOG BIT THE CAT
Adding an extra letter makes it:THH EDO GBI TTH ECA T
It changes the entire sentence to nonsense. This kind of mutation has a more dramatic effect on the DNA sequence and is usually lethal